Method and Apparatus for Generating Input Images for Holographic Waveguide Displays

ABSTRACT

An image generation device comprises: a spatial light modulator; a source of light; a beam deflector; an illumination waveguide and an image transport waveguide, each waveguide containing at least one switchable grating; and a coupler for directing scanned light into a first set of TIR paths in said illumination waveguide. A switchable grating in the illumination waveguide diffracts light onto said SLM, a switchable grating in said image transport waveguide diffracting image-modulated from the SLM into a waveguide path.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/360,491, entitled “Method and Apparatus for Generating Input Imagesfor Holographic Waveguide Displays,” filed Mar. 21, 2019, which is acontinuation of U.S. patent application Ser. No. 15/512,500, entitled“Method and Apparatus for Generating Input Images for HolographicWaveguide Displays,” filed Mar. 17, 2017, which is a U.S. National Phaseof PCT Application No. PCT/GB2015/000203, entitled “Method and Apparatusfor Generating Input Images for Holographic Waveguide Displays,” filedJun. 29, 2015, which claims the priority of U.S. Provisional PatentApplication No. 62/071,277, entitled “Method and Apparatus forGenerating Input Images for Holographic Waveguide Displays,” filed Sep.19, 2014, the disclosures of which are incorporated herein by referencein their entireties.

BACKGROUND OF THE INVENTION

This invention relates to an apparatus for generating an image, and moreparticularly to an apparatus for generating an input image for aholographic waveguide display.

The invention addresses the problem of providing uniform outputilluminination in holographic waveguide displays. U.S. patentapplication Ser. No. 13/844,456 entitled TRANSPARENT WAVEGUIDE DISPLAY,PCT Application No.: GB2012/000677 entitled WEARABLE DATA DISPLAY, U.S.patent application Ser. No. 13/317,468 entitled COMPACT EDGE ILLUMINATEDEYEGLASS DISPLAY, U.S. patent application Ser. No. 13/869,866 entitledHOLOGRAPHIC WIDE ANGLE DISPLAY, and U.S. patent application Ser. No.13/844,456 entitled TRANSPARENT WAVEGUIDE DISPLAY all of which areincorporated herein by reference in their entireties.

The invention addresses a particular category of holographic waveguidesbased on Switchable Bragg Gratings (SBGs). SBGs are fabricated by firstplacing a thin film of a mixture of photopolymerizable monomers andliquid crystal material between parallel glass plates. One or both glassplates support electrodes, typically transparent indium tin oxide films,for applying an electric field across the film. A volume phase gratingis then recorded by illuminating the liquid material (often referred toas the syrup) with two mutually coherent laser beams, which interfere toform a slanted fringe grating structure. During the recording process,the monomers polymerize and the mixture undergoes a phase separation,creating regions densely populated by liquid crystal micro-droplets,interspersed with regions of clear polymer. The alternating liquidcrystal-rich and liquid crystal-depleted regions form the fringe planesof the grating. The resulting volume phase grating can exhibit very highdiffraction efficiency, which may be controlled by the magnitude of theelectric field applied across the film. When an electric field isapplied to the grating via transparent electrodes, the naturalorientation of the LC droplets is changed causing the refractive indexmodulation of the fringes to reduce and the hologram diffractionefficiency to drop to very low levels. Note that the diffractionefficiency of the device can be adjusted, by means of the appliedvoltage, over a continuous range. The device exhibits near 100%efficiency with no voltage applied and essentially zero efficiency witha sufficiently high voltage applied. In certain types of HPDLC devicesmagnetic fields may be used to control the LC orientation. In certaintypes of HPDLC phase separation of the LC material from the polymer maybe accomplished to such a degree that no discernible droplet structureresults. SBGs may be used to provide transmission or reflection gratingsfor free space applications. In waveguide applications the parallelglass plates used to form the HPDLC cell provide a total internalreflection (TIR) light guiding structure. Light is “coupled” out of theSBG when the switchable grating diffracts the light at an angle beyondthe TIR condition. Typically, the HPDLC used in SBGs comprise liquidcrystal (LC), monomers, photoinitiator dyes, and coinitiators. Themixture frequently includes a surfactant. The patent and scientificliterature contains many examples of material systems and processes thatmay be used to fabricate SBGs. Two fundamental patents are: U.S. Pat.No. 5,942,157 by Sutherland, and U.S. Pat. No. 5,751,452 by Tanaka etal. Both filings describe monomer and liquid crystal materialcombinations suitable for fabricating SBG devices. One of the knownattributes of transmission SBGs is that the LC molecules tend to alignnormal to the grating fringe planes. The effect of the LC moleculealignment is that transmission SBGs efficiently diffract P polarizedlight (ie light with the polarization vector in the plane of incidence)but have lower diffraction efficiency for S polarized light (ie lightwith the polarization vector normal to the plane of incidence.

Waveguides offer many features that are attractive in HMDs and HUDs.They are thin and transparent. Wide fields of views can be obtained byrecording multiple holographs and tiling the field of view regionsformed by each hologram. A key feature of these waveguides is that theyprovide pupil expansion in two orthogonal directions. The pupilexpansion in a given direction is achieved by diffracting equal amountsof light out of output grating toward the eye box at each beam gratinginteraction. Uniformity of output is achieved by designing the outputgrating to have diffraction efficiency varying from a low value near theinput end of the waveguide to a high value at the furthest extremity ofthe output grating. The inventors refer to grating having suchproperties as lossy gratings. The diffraction efficiency profile alongthe waveguide may be controlled by varying one or both of the gratingrefractive index modulation and the grating thickness. According to thetheory of Bragg gratings higher index modulations give higher peakefficiency and narrow diffraction efficiency angular bandwidths.Reducing the thickness of the grating leads to a decrease in thediffraction efficiency and a broadening of the diffraction efficiencyangular bandwidth. The input image data is provided by a microdisplayexternal to the waveguide. The microdisplay which is usually areflective array must be illuminated via a beam splitter. The reflectedimage light is collimated such that each pixel of the image provides aparallel beam in a unique direction. Finally, the image light must becoupled efficiently into the waveguide so that the image content can bytransferred to the waveguide components used for orthogonal pupilexpansion. The image light from the microdisplay is normally coupledinto the waveguide via an input grating. Alternatively a prism may beused.

A major design challenge is coupling the image content into thewaveguide efficiently and in such a way the waveguide image is free fromchromatic dispersion and brightness non uniformity. To overcomechromatic dispersion and to achieve the best possible collimation it isdesirable to use lasers. However, lasers and other narrow band sourcessuch as LEDs suffer from the problem of pupil banding artifacts whichmanifest themselves as output illumination non uniformity. Bandingartifacts are formed when the collimated pupil is replicated (expanded)in a TIR waveguide. In very basic terms the light beams diffracted outof the waveguide each time the beam interacts with the grating have gapsor overlaps. This leads to an illumination ripple. The degree of rippleis a function of field angle, waveguide thickness, and aperturethickness. The inventors have found by experiment and simulation thatthe effect of banding can be smoothed by dispersion with broadbandsources such as LEDs. The effects are therefore most noticed innarrowband (e.g. laser) illumination sources.

There is a requirement for an image generator for illuminating amicrodisplay, collimating the reflected image light from themicrodisplay and efficiently coupling it into a thin holographicwaveguide with high efficiency, with low chromatic dispersion and withhigh illumination uniformity.

There is a further requirement for a waveguide display comprising animage generator for illuminating a microdisplay, collimating thereflected image light from the microdisplay and efficiently coupling itinto a thin holographic waveguide with high efficiency, with lowchromatic dispersion and with high illumination uniformity, andholographic waveguides for providing pupil expansion in orthogonaldirections.

SUMMARY OF THE INVENTION

It is a first object of the invention to provide an image generator forilluminating a microdisplay, collimating the reflected image light fromthe microdisplay and efficiently coupling it into a thin holographicwaveguide with high efficiency, with low chromatic dispersion and withhigh illumination uniformity.

It is a second object of the invention to provide a waveguide displaycomprising an image generator for illuminating a microdisplay,collimating the reflected image light from the microdisplay andefficiently coupling it into a thin holographic waveguide with highefficiency, with low chromatic dispersion and with high illuminationuniformity, and holographic waveguides for providing pupil expansion inorthogonal directions.

The objects of the invention are met in a first embodiment in whichthere is provided an image generation device comprising: a spatial lightmodulator (SLM); a source emitting first wavelength light; a beamdeflector for forming the light into a scanned beam; an illuminationwaveguide containing at least one switchable grating disposed in atleast one layer; an image transport waveguide containing at least oneswitchable grating disposed in at least one layer; and a coupler fordirecting the scanned beam into a first set of TIR paths in theillumination waveguide. The at least one switchable grating in theillumination waveguide diffracts light out of the first set of TIR pathsonto the SLM. The at least one switchable grating in the image transportwaveguide diffracts image-modulated from the SLM into a second set ofTIR paths in the image transport waveguide.

In one embodiment the extent of the at least one grating along the imagetransport waveguide defines a coupling aperture, wherein the couplingaperture defines a numerical aperture for each pixel of the SLM.

In one embodiment at least one grating in the illumination waveguide andat least one grating in the image transport waveguide are switched intotheir diffracting states simultaneously.

In one embodiment the switchable grating in the illumination waveguidecomprises an array of elements each having a unique angular diffractioncharacteristic.

In one embodiment the switchable grating in the image transportwaveguide comprises an array of elements each having a unique angulardiffraction characteristic.

In one embodiment regions of the SLM are illuminated cyclically theregions being updated with new image information in phase with theillumination, wherein the at least one grating in the illuminationwaveguide and the at least one grating in the image transport waveguideare switched synchronously with the SLM image formation updates.

In one embodiment a collimating lens is disposed between theillumination waveguide and the image transport waveguide.

In one embodiment the apparatus further comprises a source emittingsecond wavelength light wherein the illumination grating waveguide andthe image transport waveguide each contain at least one grating fordiffracting the first wavelength light and at least one grating fordiffraction second wavelength light, wherein the SLM displays firstwavelength image information when the first wavelength diffractinggratings are in their diffracting states, wherein the SLM displayssecond wavelength image information when the second wavelengthdiffracting gratings are in their diffracting states.

In one embodiment the image transfer waveguide provides an optical pathto a waveguide display device.

In one embodiment further comprises a lossy grating operative to providea spatial variation of intensity across the beam.

In one embodiment the coupler comprises a prism or grating.

In one embodiment the SLM, the illumination waveguide and the imagetransport waveguide are configured in a stack.

In one embodiment the apparatus further comprises despeckler.

In one embodiment the switchable grating layers are recorded in one of aHPDLC reverse mode HPDLC, uniform modulation.

In one embodiment the source is a laser or LED.

In one embodiment the SLM is a liquid crystal device.

In one embodiment of the invention there is provided a method ofgenerating a wave guided image comprising the steps of:

-   a) providing a SLM, a illumination waveguide containing at least one    switchable grating layer, an image transport waveguide containing at    least one switchable grating layer, a laser module, a micro mirror    and a coupler;-   b) the micro-mirror sweeping light from the laser module, through a    defined angular range the light being injected into the illumination    waveguide;-   c) switching a SBG layer in the illumination waveguide to diffract    light onto the SLM;-   d) switching a SBG layer in the image transport waveguide to    diffract light reflected from the SLM into a waveguide path in the    image transport waveguide.

In one embodiment of the invention there is provide a method ofgenerating a wave guided image wherein at least one grating in theillumination waveguide and at least one grating in the image transportwaveguide are switched into their diffracting states simultaneously.

In one embodiment there is provided a method of generating a wave guidedimage comprising the steps of:

-   a) providing a SLM, a illumination waveguide containing at least one    switchable grating layer, an image transport waveguide containing at    least one switchable grating layer, a laser module, a micro mirror    and a coupler;-   b) the micro-mirror sweeping light from the laser module through a    defined angular range the light being injected into the illumination    waveguide;-   c) sequentially update regions of the SLM with image information;-   d) switching a SBG layer in the illumination waveguide to diffract    light onto a region of the SLM.-   e) switching a SBG layer in the image transport waveguide to    diffract light reflected from the SLM into a waveguide path in the    image transport waveguide.

In one embodiment there is provide a method of generating a wave guidedimage wherein simultaneously at least one grating in the illuminationwaveguide is switched into a first diffracting state and least onegrating in the image transport waveguide is switched into a seconddiffracting state and a region of the SLM is updated with imageinformation.

A more complete understanding of the invention can be obtained byconsidering the following detailed description in conjunction with theaccompanying drawings, wherein like index numerals indicate like parts.For purposes of clarity, details relating to technical material that isknown in the technical fields related to the invention have not beendescribed in detail.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross section view of an image generator in oneembodiment of the invention.

FIG. 2 is a schematic cross section view of the image generator of FIG.1 illustrating its operation in a pupil switching mode in one embodimentof the invention.

FIG. 3 is a schematic cross section view of an image generator in oneembodiment of the invention.

FIG. 4 is a schematic cross section view of the image generator of FIG.3 illustrating its operation in an aperture switching mode in oneembodiment of the invention.

FIG. 5 is a schematic cross section view of an image generatorincorporating a waveguide despeckler in one embodiment of the invention.

FIG. 6 is a flow chart illustrated a method of generating an image usinga pupil switching mode.

FIG. 7 is a flow chart illustrated a method of generating an image usingan aperture switching mode.

DETAILED DESCRIPTION OF THE INVENTION

The invention will now be further described by way of example only withreference to the accompanying drawings. It will apparent to thoseskilled in the art that the present invention may be practiced with someor all of the present invention as disclosed in the followingdescription. For the purposes of explaining the invention well-knownfeatures of optical technology known to those skilled in the art ofoptical design and visual displays have been omitted or simplified inorder not to obscure the basic principles of the invention. Unlessotherwise stated the term “on-axis” in relation to a ray or a beamdirection refers to propagation parallel to an axis normal to thesurfaces of the optical components described in relation to theinvention. In the following description the terms light, ray, beam anddirection may be used interchangeably and in association with each otherto indicate the direction of propagation of light energy alongrectilinear trajectories. Parts of the following description will bepresented using terminology commonly employed by those skilled in theart of optical design. It should also be noted that in the followingdescription of the invention repeated usage of the phrase “in oneembodiment” does not necessarily refer to the same embodiment.

In one embodiment the apparatus comprises a microdisplay 1, illuminationwaveguide layer 2, an image transport waveguide layer which is dividedinto image transport waveguide aperture 3A and an image transportwaveguide layer 3B, The illumination waveguide layer contains a threelayer SBG 4 with the separate layers indicated by 4A,4B,4C. The imagetransport grating aperture contains a SBG layer 5 with the separatelayers indicated by 5A,5B,5C. The apparatus further comprises lasermodule 6 micro mirror 7, prism 8, a SBG module comprising lossy SBGlayer 9 sandwiched by the substrates 10,11 and a coupling prism 12abutting the illumination waveguide layer 2 for coupling scanned lightinto the illumination waveguide layer, and a collimation lens 13.

Note that although three SBG layers are used in the illuminationwaveguide layer, the invention may be applied with more or fewer SBGlayers according to the angular range requirement. The number layerswill be roughly equal to the angular range to be covered divided by thediffraction efficiency angular bandwidth of the SBGs. The micro mirrorwill have a rotation range equal to cover the required angular range tocover the angular acceptance range of the microdisplay. The collimationlens 13 has a relative aperture of typically F/2. However other relativeaperture may be used as dictated by the system requirements. The imagetransport waveguide is typically less than 1 mm in thickness. Thecoupling prism 12 is not an essential feature of the invention and couldbe replaced by a grating. The purpose of the lossy SBG 9 is to expandthe input laser beam across the height of the microdisplay pixel column.Typically this distance would be a few millimeters.

The invention provides a method and apparatus for time-sequentialaperture switching using SBGs to create a multiplicity of timedistributed spatially discrete pupil apertures in a waveguide optic thatare temporally integrated to construct a spatially homogenized expandedpupil. The temporal integration of the multiple pupils serves to reducethe contrast of banding artifacts that arise from spatially discretepupil replication across the waveguide. Perfect homogenization willnormally only be achieved for a singe field angle. Typically, this willbe at the centre of the field of view. In other regions of the field ofview bright banding or dark banding may occur depending on the fieldangle range. Pupil artifacts manifest themselves in two different waysaccording to whether the display is beyond the limits of eyeaccommodation or within the eye accommodation range. Beyond the limitsof eye accommodation: that is when the display is close to the eye, asin the case of Head Mounted Displays (HMDs) and smart eyewear (eyerelief around 30 mm or less), uniformity variations are seen as afunction of field angle. The contrast of these artifacts can vary as afunction of pupil diameter (influenced display brightness and/orsee-thru brightness). Within the eye accommodation distance pupilartifacts can be directly imaged when the eye focuses on the pupil andnot at infinity; this can occurs in Head Up Displays (HUDs) and HeadDown Displays (HDDs) where the display is several inches from the eye.This is distracting to display users, impairing the functionality of thedisplay; a user focusing on a pupil artifact will then not be focusingon the far field image projected by the display.

The invention provides two primary modes of operation based on theswitching of the micro mirror and the SBG elements in the twowaveguides. The first mode, which is illustrated in FIGS. 1-2, is pupilswitching that is aperture switching at the input to the image transportwaveguide aperture. The second mode, which is illustrated in FIGS. 3-4,is field switching, that is switching at the LCoS to control the portionof the LCoS that is illuminated at any given time. We next consider thepropagation of light through the embodiment of FIG. 1. Collimated light1000 from the laser is swept by the microscanner into a range of raydirections such as 1001. The light is then expanded by the lossy SBG toprovide the light 1002 incident on the coupling prism which enters theTIR path indicated by 1003. The function of the SBGs in the illuminationwaveguide is to illuminate the desired microdisplay column, the SBGsbeing switch in synchronism with the rotation of the micro mirror. EachSBG will have a unique diffracting characteristic design to diffract aset of angularly separated ray directions from the microscanner into arange of output angularly separated ray directions incident on the imagesurface of the microdisplay. The incident and diffracted angles from agiven SBG element are given by the Bragg equation. In FIG. 1 an activeSBG 4D element deflects the TIR beam towards a column 1A of themicrodisplay in the direction 1004. From consideration of FIG. 1 itshould be apparent that by sweeping the micro mirror through a givenangular range and selectively switching SBG elements into theirdiffracting state during the sweep time any given column of pixels willbe illuminated by a range of incidence angles. For a reflectivemicrodisplay as shown in FIG. 1 these rays are in turn reflected intoopposing angles towards the collimation lens. The extreme rays reflectedfrom the microdisplay column 1A are indicated by 1005 and 1007 whichgive rise to collimated beam represented by the extreme rays 1008,1009which enter the image transport waveguide aperture to illuminate the SBGarray 5. In FIG. 1 the SBG element 5B is in its diffracting state andall others are in their non-diffracting states. The block arrow 1012indicates the direction of scrolling of the SBG elements in synchronismwith the scanning of the beam. The ray 1008 incident on element 5B isdiffracted in to the TIR path indicated by 1010 within the imagetransport waveguide aperture and proceeds into the image transportwaveguide as the TIR light 1011. In the envisaged applications of theinvention the light 1011 would be wave guided to separate waveguidegrating elements for pupil expansion in orthogonal directions. FIG. 2illustrates the pupil switching process used in the embodiment of FIG. 1in more detail with the filed regions diffracted by the elements oftransport waveguide being indicated by the boundary rays originating atthe microdisplay column 1A indicated 1006A-1006F ad th boundarycollimated rays by 1013A-1013B.

We next consider the aperture switching mode referring to FIG. 3. Theapparatus is similar to that of FIG. 1 except that the image transportwaveguide aperture 20A now contains three SBG array layers, 21,22,23.Each stack of elements such as 21A,22A,23A receives light from threeseparate portions of the microdisplay. Only one stack diffracts at anytime. All elements of all other SBGs are in their non diffractingstates. This process is shown in more detail in FIG. 4 which shows themicrodisplay divided into the three regions 1B,1C,1D. Ray paths from thecentre column of each region to the SBG stack elements 21A,22A,23A bythe rays paths 1021-1026, which include the paths through the collimatorlens 13 and the SBG stack, are illustrated. In one embodiment all threeSBGs in the stack are in their active state simultaneously. In oneembodiment the SBGs are switched simultaneously. In one embodiment theSBGs are switched synchronously with the switching of the microdisplaycolumns such that for example: when the microdisplay columns 1B aredisplayed the element 21A is in its active state, when the microdisplaycolumns 1C are displayed the element 22A is in its active state, and soon. The captured light proceeds via TIR into the image transportwaveguide 20A.

It should be apparent from consideration of the drawings and the abovedescription that pupil switching and aperture switching can be combinedin a single image generation process. This embodiment would use the sameoptical components as the embodiments discussed above with anappropriate switching procedure for the illumination waveguide and imagetransport waveguide SBGs. The chief limitation on the switching schemewould be the switching on and off times for a SBG element.

The above described pupil switching and aperture switching schemes arecharacterised by five synchronized switching events which are describedbelow with reference to FIG. 2 and FIG. 4. In a first event themicro-mirror sweeps a laser beam, through a defined angular range thelight being injected into the illumination waveguide. The following twoevents (second and third) are concerned with pupil switching. In thesecond event the diffracting SBG grating layer in the illuminationwaveguide is switched (between the three layers). This SBG switchingcovers the angular bandwidth required to fill the full acceptance angleof the microdisplay. The angular range covered by each SBG layer andmicroscanner sweep corresponds with the image transport waveguideaperture element selected. In the third event the SBG element under theilluminated portion of the image transport waveguide aperture isswitched to a diffracting state. The following two events (fourth andfifth) are concerned with field switching. In the fourth event threedefined portions of the microdisplay are time sequentially illuminatedby SBG elements of the illumination waveguide layer. Note that only asmall field angle is illuminated at a given time. In the fifth event SBGelements in stack of SBG elements in the image transfer waveguideaperture are switching into their diffracting states in synchronism withthe switching of the SBG elements of the illumination waveguide layer.

In the embodiment of FIG. 5, which is identical to that of FIG. 1, theillumination waveguide also includes a waveguide despeckler 30. Thedespeckler is desirable a waveguide despeckler based on principlesdisclosed in PCT Application No.: PCT/GB2013/000500 entitled WAVEGUIDEFOR HOMOGENIZING ILLUMINATION, and U.S. Pat. No. 8,224,133 entitledLASER ILLUMINATION DEVICE both of which are incorporated herein byreference in their entireties.

In one embodiment illustrated in the flow diagram of FIG. 6 a method ofgenerating a image using the pupil switching scheme of the embodiment ofFIG. 1 is provided. Referring to the flow diagram, we see that the saidmethod comprises the following steps:

At step 2000 provide a SLM, a illumination waveguide layer containing atleast one switchable grating layer, an image transport waveguide layercontaining at least one switchable grating layer, an image transportwaveguide containing at least one switchable grating layer, a lasermodule, a micro mirror and a coupler;

At step 2001 the micro-mirror sweeps light from said laser module,through a defined angular range, the light being injected into theillumination waveguide

At step 2002 switch a grating layer in the illumination waveguide todiffract light onto the SLM.

At step 2003 switch a grating layer in the image transport waveguide todiffract light reflected from the SLM into a waveguide path in the imagetransport waveguide.

In one embodiment illustrated in the flow diagram of FIG. 7 a method ofgenerating a image using the pupil switching scheme of the embodiment ofFIG. 3 is provided. Referring to the flow diagram, we see that the saidmethod comprises the following steps:

At step 2010 provide a SLM, a illumination waveguide layer containing atleast one switchable grating layer, an image transport waveguide layercontaining at least one switchable grating layer, an image transportwaveguide containing at least one switchable grating layer, a lasermodule, a micro mirror and a coupler;

At step 2011 the micro-mirror sweeps light from said laser module,through a defined angular range the light being injected into theillumination waveguide

At step 2012 sequentially update regions of the SLM with imageinformation.

At step 2013 switch a grating layer in the illumination waveguide todiffract light onto a region of the SLM.

At step 2014 switch a grating layer in the image transport waveguide todiffract light reflected from the SLM into a waveguide path in the imagetransport waveguide.

It should be emphasized that the drawings are exemplary and that thedimensions have been exaggerated. For example thicknesses of the SBGlayers have been greatly exaggerated.

In any of the above embodiments the waveguides may be curved or formedfrom a mosaic of planar or curved facets.

An image generator based on any of the above-described embodiments maybe implemented using plastic substrates using the materials andprocesses disclosed in PCT Application No.: PCT/GB2012/000680, entitledIMPROVEMENTS TO HOLOGRAPHIC POLYMER DISPERSED LIQUID CRYSTAL MATERIALSAND DEVICES. Advantageously, the SBGs are recorded in a reverse modeHPDLC material in which the diffracting state of SBG occurs when anelectric field is applied across the electrodes. An eye tracker based onany of the above-described embodiments may be implemented using reversemode materials and processes disclosed in the above PCT application.

While the invention may be applied with gratings of any type includingswitching or non-switching gratings based on Bragg (volume) holograms,or surface-relief gratings the preferred grating technology is a SBG,which offers the advantages of fast switching, high optical efficiencyand transparency and high index modulation.

The method of fabricating the SBG pixel elements and the ITO electrodesused in any of the above-described embodiments of the invention may bebased on the process disclosed in the PCT Application No. US2006/043938,entitled METHOD AND APPARATUS FOR PROVIDING A TRANSPARENT DISPLAY.

It should be understood by those skilled in the art that while thepresent invention has been described with reference to exemplaryembodiments, it is to be understood that the invention is not limited tothe disclosed exemplary embodiments. Various modifications,combinations, sub-combinations and alterations may occur depending ondesign requirements and other factors insofar as they are within thescope of the appended claims or the equivalents thereof.

1.-19. (canceled)
 20. An image generation device comprising: an imagesource configured to project collimated image-modulated light of a firstwavelength over a field of view comprising a plurality of portions ofthe field of view; and a waveguide supporting a plurality of switchablegratings each switchable between a non-diffracting state and adiffracting state and configured to couple said image-modulated lightinto said waveguide, wherein said plurality of switchable gratings: areconfigured to switch into their diffracting states in synchronism withthe projection of one or more selected portions of said field of view bysaid image source, in their diffracting states, provide an aperture forcoupling light into said waveguide, are configured to provide adifferent aperture for each said field of view portion, and are recordedin a liquid crystal and polymer holographic recording material.
 21. Theimage generation device of claim 20, wherein said plurality ofswitchable gratings exhibit high diffraction efficiency with no voltageapplied and low diffraction efficiency with a voltage applied.
 22. Theimage generation device of claim 20, wherein said plurality ofswitchable gratings exhibit low diffraction efficiency with no voltageapplied and high diffraction efficiency with a voltage applied.
 23. Theimage generation device of claim 20, wherein said aperture is replicatedby total internal reflection within said waveguide.
 24. The imagegeneration device of claim 20, wherein said plurality of switchablegratings are disposed in more than one layer.
 25. The image generationdevice of claim 20, wherein said field of view portions are projected intime sequence.
 26. The image generation device of claim 20, wherein saidwaveguide is a component of a waveguide display.
 27. The imagegeneration device of claim 20, wherein said waveguide further comprisesat least one set of gratings selected from the group of: gratingsproviding beam expansion in at least one dimension, gratings providingextraction of said light from said waveguide, gratings providing fieldof view tiling, gratings with spatially varying refractive indexmodulation, and gratings with spatially varying thickness.
 28. The imagegeneration device of claim 20, wherein said plurality of switchablegratings each have a unique angular diffraction characteristic.
 29. Theimage generation device of claim 20, further comprising at least one ofa collimating lens or a despeckler.
 30. The image generation device ofclaim 20, wherein said waveguide is selected from the group consistingof a curved waveguide and a plastic waveguide.
 31. The image generationdevice of claim 20, wherein said image source employs a light sourcecomprising one of a laser or a light emitting diode.
 32. The imagegeneration device of claim 20, wherein said image source comprises: aspatial light modulator; a source emitting first wavelength light; abeam deflector for forming said light into a scanned beam; anillumination waveguide containing at least one switchable gratingdisposed in at least one layer; an image transport waveguide containingsaid at least one switchable grating disposed in at least one layer; anda coupler for directing said scanned beam into a first set of totalinternal reflection paths in said illumination waveguide, wherein saidat least one switchable grating in said illumination waveguide isconfigured to diffract light out of said first set of total internalreflection paths onto said spatial light modulator, and wherein said atleast one switchable grating in said image transport waveguide isconfigured to diffract the image-modulated light from said spatial lightmodulator into a second set of total internal reflection paths in saidimage transport waveguide.
 33. The image generation device of claim 32,wherein said at least one switchable grating defines a couplingaperture, and wherein said coupling aperture defines a numericalaperture for each pixel of said spatial light modulator.
 34. An imagegeneration device comprising: an image source configured to projectcollimated image-modulated light of a first wavelength over a field ofview comprising a plurality of portions of the field of view; and awaveguide supporting a first plurality of switchable gratings eachswitchable between a non-diffracting state and a diffracting state andconfigured to couple said image-modulated light into said waveguide,wherein said plurality of switchable gratings: are configured to switchinto their diffracting states in synchronism with the projection of oneor more selected portions of said field of view by said image source, intheir diffracting states provide an aperture for coupling light intosaid waveguide, and are configured to provide a different aperture foreach said field of view portion, and wherein said image source providesimage-modulated light of a second wavelength and said waveguide supportsa second plurality of switchable gratings for diffracting said secondwavelength image-modulated light.
 35. An image generation device ofclaim 34, wherein one or more of said first plurality of switchablegratings are switched into a diffracting state when said firstwavelength image modulated light is projected and wherein one or more ofsaid second plurality of switchable gratings are switched into adiffracting state when said second wavelength image modulated light isprojected.
 36. An image generation device comprising: an image sourceconfigured to project collimated image-modulated light of a firstwavelength over a field of view comprising a plurality of portions ofthe field of view; and a waveguide supporting a first plurality ofswitchable gratings each switchable between a non-diffracting state anda diffracting state and configured to couple said image-modulated lightinto said waveguide, wherein said plurality of switchable gratings: areconfigured to switch into their diffracting states in synchronism withthe projection of one or more selected portions of said field of view bysaid image source, in their diffracting states provide an aperture forcoupling light into said waveguide, are configured to provide adifferent aperture for each said field of view portion, and include atleast one grating selected from the group consisting of: a uniformmodulation holographic polymer dispersed liquid crystal (HPDLC) grating,a non-switching HPDLC grating, a reverse mode HPDLC grating, and asurface relief grating.